Ras genes are found to be activated in a high proportion of human tumors. The p21 proteins from these tumors have amino acid alterations at critical positions. In order to characterize the structural and functional domains of the ras genes, we have used a variety of techniques to mutate these genes. Both site-directed mutagenesis using oligonucleotides and linker insertion techniques were employed. We found that certain mutants alter the transforming ability of ras genes. When mutated proteins were analyzed for their biochemical properties, certain mutants were no longer able to autophosphorylate in an in vitro assay. We are analyzing these mutants to characterize the altered biochemical properties of p21 proteins.